Intaglio printing presses are widely used in security printing for printing security documents, especially banknotes. Prior art intaglio printing presses are for instance disclosed in Swiss Patent No. CH 477 293 A5, European Patent Publications Nos. EP 0 091 709 A1, EP 0 406 157 A1, EP 0 415 881 A2, EP 0 563 007 A1, EP 0 873 866 A1, EP 1 602 483 A1, and International Publications Nos. WO 01/54904 A1, WO 03/047862 A1, WO 2004/026580 A1, WO 2005/118294 A1, WO 2011/077348 A1, WO 2011/077350 A1, WO 2011/077351 A1, all assigned to the instant Applicant.
FIG. 1 schematically illustrates an intaglio printing press which is generally designated by reference numeral 1. More precisely, FIG. 1 shows a sheet-fed intaglio printing press 1 comprising a sheet feeder 2 for feeding sheets to be printed, an intaglio printing unit 3 for printing the sheets, and a sheet delivery unit 4 for collecting the freshly-printed sheets. The intaglio printing unit 3 includes an impression cylinder 7, a plate cylinder 8 (in this example, the plate cylinder 8 is a three-segment plate cylinder carrying three intaglio printing plates), an inking system comprising an ink-collecting cylinder, or Orlof cylinder, 9 (here a three-segment blanket cylinder carrying a corresponding number of blankets) for inking the surface of the intaglio printing plates carried by the plate cylinder 8 and an ink wiping system 10 for wiping the inked surface of the intaglio printing plates carried by the plate cylinder 8 prior to printing of the sheets.
The sheets are fed from the sheet feeder 2 onto a feeder table and then onto the impression cylinder 7. The sheets are then carried by the impression cylinder 7 to the printing nip between the impression cylinder 7 and the plate cylinder 8 where intaglio printing is performed. Once printed, the sheets are transferred away from the impression cylinder 7 for conveyance by a sheet transporting system 15 in order to be delivered to the delivery unit 4. The sheet transporting system 15 conventionally comprises a sheet conveyor system with a pair of endless chains driving a plurality of spaced-apart gripper bars for holding a leading edge of the sheets (the freshly-printed side of the sheets being oriented downwards on their way to the delivery unit 4), sheets being transferred in succession to a corresponding one of the gripper bars.
During their transport to the sheet delivery unit 4, the freshly printed sheets are preferably inspected by an optical inspection system 5. In the illustrated example, the optical inspection system 5 is advantageously an inspection system as disclosed in International Publication No. WO 2011/161656 A1, which inspection system 5 comprises a transfer mechanism and an inspection drum located at the transfer section between the impression cylinder 7 and chain wheels of the sheet transporting system 15. The optical inspection system 5 could alternatively be an inspection system placed along the path of the sheet transporting system 15 as described in International Publications Nos. WO 97/36813 A1, WO 97/37329 A1, and WO 03/070465 A1. Such inspection systems are in particular marketed by the Applicant under the product designation NotaSave®.
Before delivery, the printed sheets are preferably transported in front of a drying or curing unit 6 disposed after the inspection system 5 along the transport path of the sheet transporting system 15. Drying or curing could possibly be performed prior to the optical inspection of the sheets.
FIG. 2 is a schematic view of the intaglio printing unit 3 of the intaglio printing press 1 of FIG. 1. As already mentioned, the printing unit 3 basically includes the impression cylinder 7, the plate cylinder 8 with its intaglio printing plates, the inking system with its ink-collecting cylinder 9, and the ink wiping system 10.
The inking system comprises in this example five inking devices 20, all of which cooperate with the ink-collecting cylinder 9 that contacts the plate cylinder 8. It will be understood that the illustrated inking system is adapted for indirect inking of the plate cylinder 8, i.e. inking of the intaglio printing plates via the ink-collecting cylinder 9. The inking devices 20 each include an ink duct 21 cooperating in this example with a pair of ink-application rollers 22. Each pair of ink-application rollers 22 in turn inks a corresponding chablon cylinder 23 which is in contact with the ink-collecting cylinder 9. As is usual in the art, the surface of the chablon cylinders 23 is structured so as to exhibit raised portions corresponding to the areas of the intaglio printing plates intended to receive the inks in the corresponding colours supplied by the respective inking devices 20.
As shown in FIGS. 1 and 2, the impression cylinder 7 and plate cylinder 8 are both supported by a stationary (main) frame 50 of the printing press 1. The inking devices 20 (including the ink duct 21 and ink-application rollers 22) are supported in a mobile inking carriage 52, while the ink-collecting cylinder 9 and chablon cylinders 23 are supported in an intermediate carriage 51 located between the inking carriage 52 and the stationary frame 50. Both the inking carriage 52 and the intermediate carriage 51 are advantageously suspended under supporting rails. In FIG. 1, reference numeral 52′ designates the inking carriage in a retracted position shown in dashed lines.
The twin-carriage configuration of the intaglio printing press 1 illustrated in FIGS. 1 and 2 corresponds in essence to the configuration disclosed in International Publications Nos. WO 03/047862 A1, WO 2011/077348 A1, WO 2011/077350 A1 and WO 2011/077351 A1, all assigned to the present Applicant and which are incorporated herein by reference in their entirety.
The ink wiping system 10, on the other hand, typically comprises a wiping tank, a wiping roller assembly 11 supported on and partly located in the wiping tank and contacting the plate cylinder 8, cleaning means (not shown) for removing wiped ink residues from the surface of the wiping roller assembly 11 using a wiping solution that is sprayed or otherwise applied onto the surface of the wiping roller assembly 11, and a drying blade (not shown) contacting the surface of the wiping roller assembly 11 for removing wiping solution residues from the surface of the wiping roller assembly 11. A particularly suitable solution for the ink wiping system 10 is disclosed in International Publication No. WO 2007/116353 A1.
A particularity of intaglio printing presses as used for the production of security documents resides in the fact that very high printing pressures are applied at the printing nip between the plate cylinder and the impression cylinder, with line pressures in the range of 10'000 N/cm or more, i.e. more than 80 tons over the entire contact portion between the plate cylinder and the impression cylinder. These very high printing pressures lead to the characteristic embossing and tactile effect that is readily recognizable on intaglio-printed security documents, like banknotes.
A plate cylinder of an intaglio printing press, as for instance shown in FIGS. 1 and 2, typically consists of a cylinder base made of e.g. steel that carries one or more intaglio printing plates which are typically nickel plates (or any other suitable metal such as steel, brass or the like) whose surface is typically covered by a layer of wear-resistant material such as chromium (Cr). On the other hand, an impression cylinder of an intaglio printing press, as for instance shown in FIGS. 1 and 2, typically consists of a cylinder base made of e.g. steel which carries one or more impression blankets and one or more underlay sheets (or packing sheets) of e.g. paper or cardboard, the thickness of which is calibrated.
At the printing nip between the plate cylinder and the impression cylinder, the surface of the impression cylinder is typically compressed by the comparatively harder and more rigid surface of the plate cylinder, which leads to a local compression and deformation of the impression blanket and underlying packing sheets. The amount of compression and deformation depends on the pressure applied at the printing nip, but also on the nature and compressibility of the impression blanket and underlying packing sheets, as well as the relevant thickness of the intaglio printing plate(s) on the plate cylinder side and thickness of the impression blanket(s) and packing sheets on the impression cylinder side. It is to be further appreciated that the sheet material, which is held onto the circumference of the impression cylinder, is pressed at the printing nip between the plate cylinder and the impression cylinder.
As is typical in the art, the plate cylinder and the impression cylinder of intaglio printing presses each commonly comprise one or more cylinder pits and a corresponding number of cylinder segments, the plate cylinder and the impression cylinder coming into rolling contact with one another along their respective cylinder segments when no cylinder pits are present at the printing nip. In the example of FIGS. 1 and 2, both the plate cylinder and the impression cylinder comprise three cylinder pits 8a, resp. 7a, and three cylinder segments 8b, resp. 7b each. Three intaglio printing plates, designated by reference 8c, are mounted on the circumference of the plate cylinder 8, namely on the three cylinder segments 8b, while three sets of impression blankets and packing sheets, jointly designated by reference 7c, are mounted on the circumference of the impression cylinder 7, namely on the three cylinder segments 7b. The plate cylinder and impression cylinder may however exhibit different numbers of cylinder pits and cylinder segments (as for instance disclosed in European Patent Publication No. EP 0 873 866 A1). In any event, upon coming into rolling contact of the plate cylinder and the impression cylinder, when the trailing end of the cylinder pits leaves the printing nip, the surface of the impression cylinder starts to be compressed as a result of the pressure applied at the printing nip between the plate cylinder and impression cylinder, which compression is applied for as long as the plate cylinder and impression cylinder are in rolling contact with one another along their respective cylinder segments. This leads to considerable mechanical constraints and stresses on the components (in particular the bearing and driving components) of the intaglio printing unit, which mechanical constraints and stresses are applied throughout the duration of the contact between the cylinders. These mechanical constraints and stresses are suppressed upon interruption of the rolling contact between the plate cylinder and the impression cylinder, namely when the leading end of the cylinders pits enters the printing nip.
For all of the above reasons, intaglio printing units have to be very robust and be designed in such a way as to withstand the aforementioned huge forces and stresses that are generated during printing operations.
This being said, multiple parameters may have an impact on the operation of the intaglio printing unit of intaglio printing presses and the resulting print quality, which parameters are dependent on operative settings made by the operator of the intaglio printing press. Such operative settings in particular include the amount of pressure applied at the printing nip during printing operations (which printing pressure is typically adjustable), the type and thickness of the intaglio printing plates, the type and thickness of the impression blankets, and the type, number and thickness of the underlying packing sheets.
In that context, European Patent Publication No. EP 0 783 964 A1 merely discloses an intaglio printing press comprising means for adjusting the contact pressure between the impression cylinder and intaglio cylinder of the intaglio printing press. Similarly, European Patent Publication No. EP 2 006 095 A2 merely discloses an intaglio printing press wherein the nip pressure (contact pressure) between various cylinders and/or rollers of the intaglio printing press can be adjusted automatically.
Depending on the relevant operative settings made by the operator, operation of the intaglio printing press may potentially exceed desired and optimum operating conditions, which could lead to excessive wear and/or inadequate printing quality.
Furthermore, the operating conditions change over time, especially as a result of the alteration of the structure and properties of the impression blankets and underlying packing sheets, as well as, to a certain extent, as a result of the deformation and elongation of the intaglio printing plates.
There is therefore a need for an intaglio printing press and a method of monitoring operation of the same which can ensure that optimum and desired operating conditions of the intaglio printing press can be guaranteed and maintained over time.